1. Field of the Invention
The present invention relates to lamp holders for discharge lamps, and in particular, to lamp holders arranged to deal with high voltage.
2. Description of Related Art
Known discharge lamps employ a glass tube containing an inert gas. An electrical potential applied to electrodes at either end of the tube causes a discharge current to flow through the tube. This discharge will produce radiation that may or may not be in the visible range. Commonly, a fluorescent coating will line the inside of the glass tube to convert the radiation into visible light. Examples of such discharge lamps are commonly known as fluorescent lamps or neon lights (although these neon lights do not necessarily contain neon gas). A large discharge lamp of the “neon” type is often referred to as a cold cathode lamp.
Discharge lamps will often operate with a relatively high voltage, for example 15 kV. Consequently, special precautions are implemented to avoid inappropriate arcing or corona discharge. For this reason, traditional lamp holders have been made of ceramic to take advantage ceramic's ability to sustain high temperature and voltages without breaking down. These traditional lamp holders have a cup-shaped body containing a U-shaped metal contact that can connect to an end cap of the discharge lamp.
Industry standards have specified criteria for routing high voltage wiring into a lamp holder. In general, it is desirable shield high voltage conductors from the environment. If a high voltage conductor must be exposed, however, the spacing through free air to ground ought to exceed a minimum established for the particular magnitude of voltage. UL 879 Standard for Electrode Receptacles for Gas-Tube Signs (5th edition-first impression, Aug. 14, 1981) specifies a spacing of 1½ inches for receptacles rated at 7,500 volts, which voltage is normally supplied with secondary wiring from a 15,000 volt transformer. See also, U.S. Pat. No. 2,406,145. col. 3, line 60 through col. 4, line 4.
In order to establish such spacing, traditional lamp holders have employed tubular, ceramic wire guideways to maintain this minimum spacing. See U.S. Pat. Nos. 2,208,812; 2,326,792; 2,375,807; 2,651,024; and 5,370,546.
Other commercial lamp holders have installed a traditional cup-shaped ceramic holder inside a metal junction box. A high voltage wire can then be routed through a flexible conduit that is attached in a conventional manner to an opening in the side of the junction box. Therefore, the high voltage wire and other high voltage components will be shielded by the flexible conduit and by the metal junction box. Any arcing or corona will be shunted to the junction box, which is typically grounded. These known ceramic holders protrude through the top of the junction box. Because the ceramic holders are not as wide as the junction box, gaps are reduced by placing atop the junction box a cover with a custom cutout designed to closely encircle the body of the ceramic holder. These designs have employed the traditional tubular ceramic spacer, but the spacer itself consumes significant space inside the junction box. For this reason, the ceramic spacer has been positioned to extend outside the box into a fitting attached to the side of the junction box.
A lamp holder disclosed in U.S. Pat. No. 5,603,627 also mounts a cup-shaped ceramic body inside a metal junction box, but eliminates the tubular ceramic spacer. Instead of a spacer, this arrangement seals the high voltage wires to a hole in the ceramic body with a silicon caulk. This holder, while fitting more easily into a metal junction box, sacrifices the shielding effect offered by the spacer. In any event, these known arrangements require the installer to keep a supply of custom covers.
See also U.S. Pat. Nos. 602,966; 1,875,179; 2,045,229; 2,620,372; 2,644,027; 3,753,027; and 5,390,094;
Accordingly, there is a need for a lamp holder with an insulating body that can shield high voltage components and still, if desired, fit easily and simply into a metal junction box.